Refractory metal nitride capped electrical contact and method for frabricating same

ABSTRACT

According to one disclosed embodiment, an electrical contact for use on a semiconductor device comprises an electrode stack including a plurality of metal layers and a capping layer formed over the plurality of metal layers. The capping layer comprises a refractory metal nitride. In one embodiment, a method for fabricating an electrical contact for use on a semiconductor device comprises forming an electrode stack including a plurality of metal layers over the semiconductor device, and depositing a refractory metal nitride capping layer of the electrode stack over the plurality of metal layers. The method may further comprise annealing the electrode stack at a temperature of less than approximately 875° C. In some embodiments, the method may additionally include forming one of a Schottky metal layer and a gate insulator layer between the electrode stack and the semiconductor device.

DEFINITION

In the present application, “group III-V semiconductor” refers to acompound semiconductor that includes at least one group III element andat least one group V element, such as, but not limited to, galliumnitride (GaN), gallium arsenide (GaAs), aluminum gallium arsenide(AlGaAs), indium gallium nitride (InGaN) and the like. Analogously,“III-nitride semiconductor” refers to a compound semiconductor thatincludes nitrogen and at least one group III element, such as, but notlimited to, GaN, AlGaN, InN, AlN, InGaN, InAlGaN and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of semiconductor devicefabrication. More specifically, the present invention is in the field offabrication of compound semiconductor devices.

2. Background Art

It is generally desirable to merge the enhanced power handlingcapabilities of group III-V semiconductor power devices, such asIII-nitride power transistors, and the energy efficiency and ease offabrication of lower power silicon, or other group IV semiconductordevices, on a common die. However, traditional techniques forfabricating III-nitride power transistors make monolithic integration ofthose devices with commonly used silicon devices quite challenging.

For example, III-nitride power semiconductor device fabricationtypically includes forming electrical contacts, such as ohmicsource/drain contacts, having a low Specific Linear Contact Resistivity(SCLR). Conventional approaches to implementing electrical contactsdisplaying suitably low SCLR values on III-nitride devices have utilizedaluminum as part of an electrode stack, and a noble metal, such as goldto form a capping layer of the stack. As a specific example, anelectrical contact stack comprising pure films of titanium, aluminum,and nickel, capped with gold, has been widely used.

This conventional approach entails several significant drawbacks,however. One drawback is that use of gold as a capping layer is costly.Another is that gold has been found to diffuse through the electrodestack, as well as capping it, so that some gold undesirably appears atthe interface of the electrical contact and the III-nitridesemiconductor body. Yet another significant drawback to the use of goldas a capping layer is its propensity to contaminate a siliconfabrication process flow. Consequently, despite its favorablecontribution to desirable SCLR values, the conventional approach tousing gold as a capping layer for electrical contacts formed onIII-nitride power semiconductor devices makes it particularly difficultto integrate those devices with silicon devices.

Thus, there is a need to overcome the drawbacks and deficiencies in theart by providing an electrical contact for use on a group III-Vsemiconductor device that renders integration of group III-V and groupIV semiconductor devices more efficient and cost effective by posing areduced contamination risk to silicon, or other group IV semiconductorfabrication process flows.

SUMMARY OF THE INVENTION

A refractory metal nitride capped electrical contact and method forfabricating same, substantially as shown in and/or described inconnection with at least one of the figures, as set forth morecompletely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a group III-V semiconductor deviceincluding a plurality of refractory metal nitride capped electricalcontacts, according to one embodiment of the present invention.

FIG. 2 is a more detailed block diagram showing a refractory metalnitride capped electrical contact, according to one embodiment of thepresent invention.

FIG. 3 is a flowchart presenting a method for fabricating a refractorymetal nitride capped electrical contact, according to one embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a refractory metal nitride cappedelectrical contact and a method for its fabrication. More specifically,the present invention discloses such a contact for implementation as anelectrode on a semiconductor device, such as a group III-V powersemiconductor device, for example. Although the present inventiveconcepts are described with respect to specific embodiments, theprinciples of the invention, as defined by the claims appended herein,can obviously be applied beyond the specifically described embodimentsof the invention described herein. Moreover, in the description of thepresent invention, certain details have been left out in order to notobscure the inventive aspects of the invention. The details left out arewithin the knowledge of a person of ordinary skill in the art.

The drawings in the present application and their accompanying detaileddescription are directed to merely example embodiments of the invention.To maintain brevity, other embodiments of the invention, which use theprinciples of the present invention, are not specifically described inthe present application and are not specifically illustrated by thepresent drawings. It should be borne in mind that, unless notedotherwise, like or corresponding elements among the figures may beindicated by like or corresponding reference numerals.

FIG. 1 is a block diagram showing an example group III-V semiconductordevice including a plurality of refractory metal nitride cappedelectrical contacts, according to one embodiment of the presentinvention. Structure 100, in FIG. 1, shows a specific implementation ofthe present inventive concepts. It should be understood that particulardetails such as the materials used to form structure 100 and thesemiconductor device represented by structure 100, for example, areprovided for conceptual clarity, and should not be interpreted aslimitations.

As shown in FIG. 1, structure 100 comprises a group III-V semiconductorhigh electron mobility transistor (HEMT) implemented in gallium nitride(GaN). Structure 100 includes substrate 102, transition structure 104,and GaN body 110 comprising GaN layer 112 and aluminum gallium nitride(AlGaN) layer 114. Structure 100 also includes a plurality of electricalcontacts including source electrode 136, drain electrode 138, andinsulated gate electrode 132. Further shown in FIG. 1 is two-dimensionalelectron gas (2DEG) 116, which provides a conduction channel for thecharge carriers of the HEMT, and is generated at the heterojunctionformed by the interface of GaN layer 112 and AlGaN layer 114, in GaNbody 110.

Substrate 102 may comprise any commonly utilized substrate material forGaN, such as sapphire, silicon, or silicon carbide, for example. Asshown in FIG. 1, in the present embodiment, GaN body 110 is formed overtransition structure 104, which is itself formed over substrate 102. Itis noted that the present embodiment is merely one representation of agroup III-V semiconductor device, however, and in other embodiments,transition structure 104 may not be utilized. For example, wheresubstrate 102 is a suitable native substrate for GaN layer 112, GaNlayer 112 may be formed on substrate 102, eliminating transitionstructure 104 entirely.

Where, however, as in the present embodiment, transition structure 104is used, transition structure 104 may correspond to a plurality ofdistinguishable layers mediating the lattice transition from substrate102 to GaN layer 112. Transition structure 104 may include, for example,a series of AlGaN layers comprising progressively less aluminum nitrideand more gallium nitride, until a suitable transition to GaN layer 112is achieved.

Source electrode 136 and drain electrode 138 are power electrodesohmically coupled to the heterojunction formed by GaN layer 112 andAlGaN layer 114. Moreover, insulated gate electrode 132 is situatedbetween source electrode 136 and drain electrode 138, on GaN body 110.As shown in FIG. 1, each of source electrode 136, drain electrode 138,and insulated gate electrode 132 comprises electrode stack 120.Electrode stack 120 includes a refractory metal nitride capping layer(not explicitly shown in FIG. 1), such as a titanium nitride (TiN)capping layer, for example. As further shown in FIG. 1, insulated gateelectrode 132 includes gate insulator layer 134, such as a siliconnitride (Si₃N₄) or silicon dioxide (SiO₂) layer, for example, underlyingelectrode stack 120. It is noted that although in the embodiment of FIG.1, gate electrode 132 is shown as an insulated gate electrode, in otherembodiments, gate electrode 132 comprising electrode stack 120 may makeSchottky contact with GaN body 110.

Turning now to FIG. 2, FIG. 2 is a more detailed block diagram showing arefractory metal nitride capped electrical contact, according to oneembodiment of the present invention. FIG. 2 shows electrical contact 230comprising electrode stack 220. Electrical contact 230 may be seen tocorrespond to any of source electrode 136, drain electrode 138, orinsulated gate electrode 132, in FIG. 1. Electrode stack 220, whichcorresponds to electrode stack 120 in FIG. 1, includes first titanium(Ti) layer 222, aluminum (Al) layer 224, second Ti layer 226, and TiNcapping layer 228.

More generally, electrode stack 220 corresponds to an electrode stackincluding a plurality metal layers, e.g., first Ti layer 222, Al layer224, and second Ti layer 226, and a capping layer comprising arefractory metal nitride. The expression “refractory metal” may beinterpreted in more than one way, as is commonly acknowledged in theart. For the purposes of the present application, the wider definitionof refractory metal to include the elements Ti, zirconium (Zr), andvanadium (Vn), as well as the traditionally recognized refractory metalsniobium (Nb), molybdenum (Mo), tantalum (Ta), and tungsten (W), isadopted. Thus, “refractory metal nitride” may refer to any of TiN, TaN,ZrN, VN, NbN, MoN, and WN₂, or any combination of those refractorynitride compounds.

Moreover, the number and character of the plurality of metal layers 222,224, and 226 of electrode stack 220 are merely exemplary. As a result,in other embodiments, electrode stack 220 may comprise more, or fewer,metal layers underlying the refractory metal nitride capping layer,e.g., TiN capping layer 228, and the plurality of metal layers may bedifferent from those shown in FIG. 2. For example, second Ti layer 226may in other embodiments correspond to a nickel (Ni) layer, while insome embodiments Al layer 224 may further comprise silicon (Si) up toapproximately ten percent (10%) by weight.

The present inventive concepts will be further described by reference toflowchart 300, in FIG. 3, which describes the steps, according to oneembodiment of the present invention, of a method for fabricating arefractory metal nitride capped electrical contact. It is noted thatcertain details and features have been left out of flowchart 300 thatare apparent to a person of ordinary skill in the art. For example, astep may comprise one or more substeps or may involve specializedequipment or materials, as known in the art. While steps 310 through 360indicated in flowchart 300 are sufficient to describe one embodiment ofthe present invention, other embodiments of the invention may utilizesteps different from those shown in flowchart 300.

Beginning with step 310 of flowchart 300 and referring to FIGS. 1 and 2,step 310 of flowchart 300 comprises forming first Ti layer 222 ofelectrode stack 120 over GaN body 110 formed over substrate 102. Anysuitable deposition process may be used to form first Ti layer 222. Forexample, first Ti layer 222 may be sputter deposited over GaN body 110.Alternatively, Ti layer 222 may formed using a chemical vapor deposition(CVD) or electron beam deposition process, for example. It iscontemplated that first Ti layer 222 is formed to a thickness ofapproximately 100 Å, plus or minus approximately ten percent (+/−10%).Although the present embodiment refers to formation of first Ti layer222 over GaN body 110, it is emphasized that GaN body 110 may correspondto a body formed from any combination of suitable group III-Vsemiconductor materials, as described in the “Definition” section above.

Typically, the only constraint placed upon the composition of GaN body110 is that it comprise at least one layer of a group III-Vsemiconductor material. In some embodiments, as shown in FIG. 1, GaNbody 110 may comprise a first group III-V semiconductor layer, e.g., GaNlayer 112, and a second group III-V semiconductor layer formed over thefirst III nitride semiconductor layer, e.g., AlGaN layer 114 formed overGaN layer 112, for example, wherein the second group III-V semiconductorlayer comprises a group III-V semiconductor having a different, e.g.,wider, band gap than the group III-V semiconductor forming the firstgroup III-V semiconductor layer.

Continuing with step 320 of FIG. 3, step 320 of flowchart 300 comprisesforming Al layer 224 of electrode stack 120 over first Ti layer 222 ofelectrode stack 120. As was the case for formation of first Ti layer222, in step 310, formation of Al layer 224 in step 320 may proceedusing any suitable deposition technique, such as sputter deposition,CVD, or electron beam deposition. As shown in FIG. 2, in the presentembodiment, Al layer 224 is formed to a greater thickness than first Tilayer 222. Al layer 224 may be formed to a thickness of approximately1300 Å, plus or minus approximately ten percent (+/−10%), for example.

Moving to step 330 of flowchart 300, step 330 comprises forming secondTi layer 226 of electrode stack 120 over Al layer 224 of electrode stack120. Once again, formation of second Ti layer 226 in step 330 mayproceed using any suitable deposition technique, such as sputterdeposition, CVD, or electron beam deposition. As shown in FIG. 2, in thepresent embodiment, second Ti layer 226 is formed to a thickness greaterthan that of first Ti layer 222, and less than that of Al layer 224, butthat representation is merely exemplary. In the present embodiment, forexample, second Ti layer 226 may be formed to a thickness ofapproximately 680 Å, plus or minus approximately ten percent (+/−10%).

Continuing with step 340 of flowchart 300, step 340 comprises depositingbiased reactive TiN capping layer 228 over second Ti layer 226. As iswell known in the art, a species of TiN having a particularly highdensity, i.e., “biased reactive TiN” (also referred to in the art as“gold” TiN), can be formed by deposition of TiN onto a body supported bya substrate to which an appropriate biasing voltage is applied, in thepresence of nitrogen gas with which the TiN reacts. Thus, in oneembodiment, substrate 102 supporting GaN body 110, in FIG. 1, mayreceive a biasing voltage during deposition of TiN capping layer 228 ofelectrode stack 120, in a deposition chamber to which nitrogen gas hasbeen introduced.

As shown in FIG. 2, in the present embodiment, TiN capping layer 228 isformed to a thickness comparable to that of second Ti layer 226. Forexample, TiN capping layer 228 may be formed to a thickness ofapproximately 600 Å, plus or minus approximately ten percent (+/−10%).As previously mentioned, other embodiments may employ a differentrefractory metal nitride capping layer in place of TiN capping layer228. For instance, in one embodiment, refractory metal nitride cappingof electrode stack 220 may be performed by a TaN capping layercorresponding to TiN capping layer 228.

In step 350 of flowchart 300, TiN capping layer 228, second Ti layer226, Al layer 224, and first Ti layer 222 may be patterned and plasmaetched to form one or more of electrode stacks 120 over GaN body 110.Subsequently, in step 360, electrode stacks 120 are annealed atapproximately 850° C. for approximately sixty seconds to produce one ormore electrical contacts, e.g., source contact 136 and/or drain contact138, on GaN body 110. In some embodiments, annealing of electrode stacks120, in step 360, may be performed in the presence of nitrogen gas.

Thus, according to the present invention, the novel refractory metalnitride capped electrical contact described herein presents significantadvantages over the conventional art, such as rendering integration ofgroup III-V and group IV semiconductor devices more efficient and costeffective by posing a reduced contamination risk to silicon, or othergroup IV semiconductor fabrication process flows.

From the above description of the invention it is manifest that varioustechniques can be used for implementing the concepts of the presentinvention without departing from its scope. Moreover, while theinvention has been described with specific reference to certainembodiments, a person of ordinary skill in the art would recognize thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. The described embodiments are to beconsidered in all respects as illustrative and not restrictive. Itshould also be understood that the invention is not limited to theparticular embodiments described herein, but is capable of manyrearrangements, modifications, and substitutions without departing fromthe scope of the invention.

The invention claimed is:
 1. An electrical contact for use on asemiconductor device, said electrical contact comprising: an electrodestack including a plurality of metal layers and a capping layer formedover said plurality of metal layers; said capping layer consisting onlyof a refractory metal nitride, said refractory metal nitride forming atop of said electrical contact and having a thickness greater than thatof a bottommost layer of said plurality of metal layers.
 2. Theelectrical contact of claim 1, wherein said capping layer comprisestitanium nitride (TiN).
 3. The electrical contact of claim 1, whereinsaid capping layer comprises tantalum nitride (TaN).
 4. The electricalcontact of claim 1, wherein said plurality of metal layers includesrespective first, second, and third metal layers, wherein said secondmetal layer is formed between said first metal layer and said thirdmetal layer, and said capping layer is formed on said third metal layer.5. The electrical contact of claim 4, wherein said first metal layer andsaid third metal layer of said electrode stack comprise titanium (Ti).6. The electrical contact of claim 4, wherein said second metal layer ofsaid electrode stack comprises aluminum (Al).
 7. The electrical contactof claim 6, wherein said second metal layer of said electrode stackfurther comprises silicon (Si).
 8. The electrical contact of claim 1,wherein said electrical contact is configured for implementation on agroup III-V semiconductor device.
 9. The electrical contact of claim 8,wherein said group III-V semiconductor device comprises a high electronmobility transistor (HEMT).
 10. The electrical contact of claim 8,wherein said electrical contact is implemented as a power electrodemaking ohmic contact with said group III-V semiconductor device.
 11. Theelectrical contact of claim 8, wherein said electrical contact isimplemented as a gate electrode making Schottky contact with said groupIII-V semiconductor device.
 12. The electrical contact of claim 8,wherein said electrical contact is implemented with a gate insulatorlayer underlying said first metal layer of said electrode stack toprovide an insulated gate electrode for said group III-V semiconductordevice.
 13. An electrical contact for use on a group III-V semiconductordevice, said electrical contact comprising: an electrode stack includinga plurality of metal layers and a capping layer formed over saidplurality of metal layers; said capping layer consisting only of a metalselected from the group consisting of titanium nitride (TiN) andtantalum nitride (TaN), said metal forming a top of said electricalcontact and having a thickness greater than that of a bottommost layerof said plurality of metal layers.
 14. The electrical contact of claim13, wherein said group III-V semiconductor device comprises a highelectron mobility transistor (HEMT).
 15. The electrical contact of claim13, wherein said electrical contact is implemented as a power electrodemaking ohmic contact with said group III-V semiconductor device.
 16. Theelectrical contact of claim 13, wherein said electrical contact isimplemented as a gate electrode making Schottky contact with said groupIII-V semiconductor device.
 17. The electrical contact of claim 13,wherein said electrical contact is implemented with a gate insulatorlayer underlying a first metal layer of said electrode stack to providean insulated gate electrode for said group III-V semiconductor device.